search for dark or light mode stylesheet

summary:
-------
This commit adds xdg-desktop-portal support to waybar. If a portal
supporting `org.freedesktop.portal.Settings` exists, then it will be
queried for the current colorscheme. This colorscheme will then be used
to prefer a `style-light.css` or `style-dark.css` over the basic
`style.css`.

technical details:
-----------------
Appearance is provided by several libraries, such as libhandy (mobile)
and libadwaita. However, waybar links to neither of these libraries. As
the amount of code required to communicate with xdg-desktop portal as a
client is rather minimal, I believe doing so is better than linking to
an additional library.

The Gio library for communicating with dbus is rather messy, Instead of
the `Portal` class containing a `Gio::Dbus::Proxy`, it extends it which
simplifies signal handling.

`Portal` then exposes its own signal, which can be listened to by waybar
to update CSS.

For a reference implementation, please see another one of my projects:
https://github.com/4e554c4c/darkman.nvim/blob/main/portal.go

test plan:
---------
If no desktop portal which provides `Settings` exists, then waybar
continues with the log line
```
[2023-09-06 14:14:37.754] [info] Unable to receive desktop appearance: GDBus.Error:org.freedesktop.DBus.Error.UnknownMethod: No such interface “org.freedesktop.portal.Settings” on object at path /org/freedesktop/portal/desktop
```

Furthermore, if `style-light.css` or `style-dark.css` do not exist, then
`style.css` will still be searched for.

Waybar has been tested with both light and dark startup. E.g. if the
appearance is dark on startup the log lines
```
[2023-09-06 14:27:45.379] [info] Discovered appearance 'dark'
[2023-09-06 14:27:45.379] [debug] Try expanding: $XDG_CONFIG_HOME/waybar/style-dark.css
[2023-09-06 14:27:45.379] [debug] Found config file: $XDG_CONFIG_HOME/waybar/style-dark.css
[2023-09-06 14:27:45.379] [info] Using CSS file /home/pounce/.config/waybar/style-dark.css
```
will be observed.
If the color then changes to light during the operation of waybar, it
will change css files:
```
[2023-09-06 14:28:17.173] [info] Received new appearance 'dark'
[2023-09-06 14:28:17.173] [debug] Try expanding: $XDG_CONFIG_HOME/waybar/style-light.css
[2023-09-06 14:28:17.173] [debug] Found config file: $XDG_CONFIG_HOME/waybar/style-light.css
[2023-09-06 14:28:17.173] [info] Using CSS file /home/pounce/.config/waybar/style-light.css
```

Finally, tested resetting waybar and toggling style (works, and style is
only changed once).

fixes: Alexays/Waybar#1973

include/client.hpp

@@ -7,6 +7,7 @@
 
 #include "bar.hpp"
 #include "config.hpp"
+#include "util/portal.hpp"
 
 struct zwlr_layer_shell_v1;
 struct zwp_idle_inhibitor_v1;
@@ -33,7 +34,7 @@ class Client {
 
  private:
   Client() = default;
-  const std::string getStyle(const std::string &style);
+  const std::string getStyle(const std::string &style, std::optional<Appearance> appearance);
   void bindInterfaces();
   void handleOutput(struct waybar_output &output);
   auto setupCss(const std::string &css_file) -> void;
@@ -46,12 +47,14 @@ class Client {
   static void handleOutputDone(void *, struct zxdg_output_v1 *);
   static void handleOutputName(void *, struct zxdg_output_v1 *, const char *);
   static void handleOutputDescription(void *, struct zxdg_output_v1 *, const char *);
+  void handleAppearanceChanged(waybar::Appearance appearance);
   void handleMonitorAdded(Glib::RefPtr<Gdk::Monitor> monitor);
   void handleMonitorRemoved(Glib::RefPtr<Gdk::Monitor> monitor);
   void handleDeferredMonitorRemoval(Glib::RefPtr<Gdk::Monitor> monitor);
 
   Glib::RefPtr<Gtk::StyleContext> style_context_;
   Glib::RefPtr<Gtk::CssProvider> css_provider_;
+  std::unique_ptr<Portal> portal;
   std::list<struct waybar_output> outputs_;
 };
 

include/util/portal.hpp

@@ -0,0 +1,38 @@
+#include <giomm/dbusproxy.h>
+
+#include <string>
+
+#include "fmt/format.h"
+
+namespace waybar {
+
+using namespace Gio;
+
+enum class Appearance {
+  UNKNOWN = 0,
+  DARK = 1,
+  LIGHT = 2,
+};
+class Portal : private DBus::Proxy {
+ public:
+  Portal();
+  void refreshAppearance();
+  Appearance getAppearance();
+
+  typedef sigc::signal<void, Appearance> type_signal_appearance_changed;
+  type_signal_appearance_changed signal_appearance_changed() { return m_signal_appearance_changed; }
+
+ private:
+  type_signal_appearance_changed m_signal_appearance_changed;
+  Appearance currentMode;
+  void on_signal(const Glib::ustring& sender_name, const Glib::ustring& signal_name,
+                 const Glib::VariantContainerBase& parameters);
+};
+
+}  // namespace waybar
+
+template <>
+struct fmt::formatter<waybar::Appearance> : formatter<fmt::string_view> {
+  // parse is inherited from formatter<string_view>.
+  auto format(waybar::Appearance c, format_context& ctx) const;
+};